Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations

We present new absolute trigonometric parallaxes and relative proper motions for nine Galactic Cepheid variable stars: ℓ Car, ζ Gem, β Dor, W Sgr, X Sgr, Y Sgr, FF Aql, T Vul, and RT Aur. We obtain these results with astrometric data from Fine Guidance Sensor 1r, a white-light interferometer on the Hubble Space Telescope. We find absolute parallaxes in milliarcseconds: ℓ Car, 2.01 ± 0.20 mas; ζ Gem, 2.78 ± 0.18 mas; β Dor, 3.14 ± 0.16 mas; W Sgr, 2.28 ± 0.20 mas; X Sgr, 3.00 ± 0.18 mas; Y Sgr, 2.13 ± 0.29 mas; FF Aql, 2.81 ± 0.18 mas; T Vul, 1.90 ± 0.23 mas; and RT Aur, 2.40 ± 0.19 mas; average σπ/π = 8%. Two stars (FF Aql and W Sgr) required the inclusion of binary astrometric perturbations, providing Cepheid mass estimates. With these parallaxes we compute absolute magnitudes in V, I, K, and Wesenheit WVI bandpasses, corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Adding our previous absolute magnitude determination for δ Cep, we construct period-luminosity relations (PLRs) for 10 Galactic Cepheids. We compare our new PLRs with those adopted by several recent investigations, including the Freedman and Sandage H0 projects. Adopting our PLR would tend to increase the Sandage H0 value, but leave the Freedman H0 unchanged. Comparing our Galactic Cepheid PLR with those derived from LMC Cepheids, we find the slopes for K and WVI to be identical in the two galaxies within their respective errors. Our data lead to a WVI distance modulus for the LMC m - M = 18.50 ± 0.03, uncorrected for any metallicity effects. Applying recently derived metallicity corrections yields a corrected LMC distance modulus of (m - M)0 = 18.40 ± 0.05. Comparing our PLR to solar-metallicity Cepheids in NGC 4258 results in a distance modulus 29.28 ± 0.08 that agrees with one derived from maser studies.